24  Unit 5: Hurricanes 5E

How do hurricanes form, and why do they only occur at certain times of year?

Author

Earth & Space Science

HS-ESS2-5 Time: 5–6 Days

🌀 Anatomy of a Hurricane 🌀

25 Engage: The Investigative Phenomenon

25.1 🌊 The 2005 Atlantic Hurricane Season

In 2005, hurricanes occurred in the North Atlantic Ocean between June 1 and November 30 — the same seasonal window as every other year. Why?

The Record-Breaking 2005 Season:

  • 🌀 28 named storms — the most ever recorded at that time
  • 💪 15 hurricanes (7 of them major: Category 3+)
  • 🏚️ Hurricane Katrina devastated New Orleans (Category 5 at peak, Cat 3 at landfall)
  • 📊 Used up the entire alphabet of names — had to use Greek letters (Alpha, Beta, Gamma…)
  • 💰 Over $180 billion in damages from the full season

25.1.1 🤔 Driving Questions:

  • What energy source powers hurricanes?
  • Why do hurricanes only form over warm ocean water?
  • Why is there a specific hurricane season (June–November)?
  • Could warming oceans lead to more seasons like 2005?

25.1.2 📝 Pattern Recognition

Before we investigate, consider:

  1. Why do you think hurricanes form over oceans but not over land?
  2. Why June through November — what’s special about those months?
  3. What do you think happens to a hurricane when it hits land? Why?

26 Explore: Sea Surface Temperature & Hurricane Formation

26.1 🔬 Investigation: The Warm Water Connection

Hurricanes need warm ocean water to form. But how warm? And why? Let’s explore the data.

26.2 Sea Surface Temperatures Through the Year

26.2.1 📝 Analyzing the SST–Hurricane Connection

  1. During which months does SST exceed 26.5°C?
  2. During which months do most hurricanes form?
  3. What’s the relationship between the two?
  4. Why do you think 26.5°C is the magic number? (We’ll find out in the Explain section!)

26.3 The 2005 Season: Month by Month

🌀 The 2005 season produced FOUR Category 5 hurricanes (Dennis, Emily, Katrina, Rita, Wilma) — the most ever recorded in a single season! 🌀

27 Explain: The Hurricane Engine

27.1 ⚡ How Does a Hurricane Work?

A hurricane is essentially a giant heat engine — it converts thermal energy from warm ocean water into the kinetic energy of powerful winds. Let’s build a model of how this works.

27.2 Step 1: Warm Water Evaporation

27.2.1 💡 Key Concept: The Hurricane Heat Engine

A hurricane is powered by a positive feedback loop:

  1. Warm ocean water (≥ 26.5°C / 80°F) evaporates rapidly
  2. Water vapor rises and carries latent heat (hidden energy)
  3. As vapor rises and cools, it condenses into clouds, releasing latent heat
  4. This released heat warms the air, making it rise faster
  5. Faster rising air creates lower pressure at the surface
  6. Lower pressure pulls in more air (= stronger winds)
  7. Stronger winds cause more evaporation → back to step 2

This is why hurricanes intensify over warm water and weaken over land or cool water — they lose their energy source!

27.3 The Saffir-Simpson Scale

28 Explain: Why Is There a Hurricane Season?

28.1 📅 The Seasonality Question

Atlantic hurricane season runs June 1 – November 30, with peak activity in August–October. This isn’t random — it’s directly tied to ocean temperature cycles.

28.2 Ocean Temperature Through the Year

The ocean’s temperature follows a seasonal cycle, but it lags behind the Sun’s position because water has a very high heat capacity — it takes a long time to heat up and a long time to cool down.

28.2.1 💡 Key Concept: The Seasonal Lag Explains Hurricane Season

  • Peak solar input occurs around the summer solstice (June 21)
  • But peak ocean temperature doesn’t occur until September — a 2–3 month lag
  • This is because water has high specific heat capacity — it absorbs a lot of energy before its temperature rises
  • Hurricane season peaks in August–September–October because that’s when ocean temperatures are highest
  • The season ends in November as ocean temperatures fall below the 26.5°C threshold

28.2.2 📝 Seasonal Analysis

  1. Why does peak SST occur ~2 months after peak solar input? (Think: specific heat capacity of water)
  2. If ocean waters warm an additional 1°C due to climate change, how might that affect hurricane season? (Start earlier? End later? Both?)
  3. Could a year with unusually warm winter SST lead to earlier hurricane formation?

29 Elaborate: Warming Oceans & Stronger Hurricanes

29.1 🌡️ How Climate Change Is Changing Hurricanes

The connection between warmer oceans and hurricanes has major implications for our future. Let’s look at the evidence.

29.2 Atlantic Ocean Warming Trend

29.3 Rapid Intensification: The Growing Threat

One of the most dangerous trends is rapid intensification — when a hurricane’s wind speed increases by 35+ mph in just 24 hours. This makes evacuations nearly impossible.

29.3.1 💡 Key Concept: Warmer Oceans → Stronger Hurricanes

Research shows that climate change affects hurricanes in several ways:

Change Mechanism Evidence
Stronger peak winds More warm water → more evaporation → more latent heat release Proportion of Cat 4-5 storms increasing
More rapid intensification Deeper warm water layers → sustained energy RI events up ~150% since 1980s
More rainfall Warmer air holds ~7% more moisture per °C Hurricane Harvey: 60+ inches in TX
Longer season Warmer water above threshold for more months Recent storms forming earlier/later
Higher storm surge Sea level rise + stronger storms Sandy’s surge was amplified by ~1 ft of SLR

29.3.2 📝 Evidence-Based Reasoning

Using the data above:

  1. Calculate the approximate trend in SST anomaly from 1980 to 2024. How much has the tropical Atlantic warmed?
  2. If SST continues rising at this rate, what anomaly would you predict for 2050?
  3. How might a longer hurricane season affect NYC specifically? Consider: When do nor’easters also form?
  4. Hurricane Sandy (2012) hit NYC as a post-tropical cyclone in late October. Was this timing unusual? What does it suggest about future risks?

30 Explain: Why Hurricanes Weaken Over Land

30.1 🏗️ The Land Problem

Hurricanes weaken dramatically once they make landfall. Understanding why helps explain the fundamental energy source of these storms.

30.1.1 💡 Key Concept: Three Reasons Hurricanes Weaken Over Land

  1. No warm water = no evaporation = no latent heat = fuel supply cut off
  2. Increased surface friction from buildings, trees, mountains = winds slow down
  3. No moisture resupply = precipitation diminishes = energy release decreases

This confirms that warm ocean water is the essential energy source for hurricanes. Without it, the hurricane engine shuts down within 12–24 hours.

31 Evaluate: Hurricane Science Assessment

31.1 ✅ Show What You Know

31.1.1 🧠 Check Your Understanding

Question 1: A hurricane is fundamentally powered by:

Question 2: Atlantic hurricane season peaks in September (not June) because:

Question 3: How could a 1°C rise in average ocean temperature affect hurricanes?

31.1.2 📝 Culminating Task: Hurricane Energy Model

Create a detailed diagram + written explanation that answers:

“How did the 2005 Atlantic hurricane season produce so many powerful storms, and what role did ocean temperature play?”

Your model should include:

  1. The energy cycle of a hurricane (evaporation → condensation → heat release → wind → more evaporation)
  2. Why 26.5°C is the threshold for formation (relate to evaporation rates and latent heat)
  3. The seasonal timing and why September is the peak (thermal lag)
  4. How 2005 SST anomalies contributed to the record-breaking season
  5. A claim about the future: will seasons like 2005 become more common? Use SST trend data as evidence.

Use the vocabulary: latent heat, evaporation, condensation, specific heat capacity, sea surface temperature, rapid intensification, thermal lag

32 Summary: Key Takeaways

Concept Key Idea
Energy source Latent heat from condensation of evaporated warm ocean water
26.5°C threshold Minimum SST for enough evaporation to fuel a tropical cyclone
Feedback loop Evaporation → condensation → heat release → stronger winds → more evaporation
Hurricane season June–Nov because of thermal lag (SST peaks ~2 months after solar maximum)
Weakening over land No water → no evaporation → no fuel + more friction
Climate connection Warmer SSTs → more available energy → potentially stronger storms & longer seasons
Rapid intensification Increasing trend linked to deeper layers of warm water

Next up: You’ll bring everything together to construct an evidence-based argument about the future of storms in your region. 📢